Method for the catalytic purification of light hydrocarbons

ABSTRACT

The invention relates to a method for the purification of light hydrocarbons having a cut point of between 20 and 250° C. and containing sulphur and/or nitrogen compounds which are refractory to standard hydrotreating. The inventive method is characterised in that it comprises the following steps, namely: (a) a step involving the oxidative polymerisation of compounds containing a —X—CH═ group in a 5- or 6 membered hydrocarbon ring, wherein X denotes a sulphur or nitrogen atom, in the presence of at least one oxidising agent selected from metal cations; (b) a step involving the separation of the formed polymers and the oxidising agent from the light hydrocarbons; and (c) a step involving the oxidation of the metal cation, said steps being performed in the above order. Moreover, each of the aforementioned steps can be combined with at least the step following same.

The present invention relates to a method of purification of lighthydrocarbons containing sulfur compounds and/or nitrogen compounds whichare refractory to the usual catalytic hydrofining treatments, such asthiophene compounds and compounds of the pyrrole type, by oxidativepolymerization of these compounds. It also relates to the regenerationand reactivation of the oxidizing agent used in this method. This methodis intended more particularly for the treatment of gasolines, notablygasolines obtained from catalytic cracking, and hydrocarbons obtainedfrom steam cracking of naphthas containing refractory compounds.

By “compounds that are refractory to the usual catalytic hydrofiningtreatments”, we mean thiophene, benzothiophene and their alkylatedderivatives, as well as pyrrole and its derivatives, benzopyrrole andcarbazole, optionally alkylated.

Usually, appreciable amounts of these thiophenic gasolines are added tomore-desulfurized gasolines from direct distillation, which are sold infilling station networks, and it is essentially these thiophenecompounds contained in the thiophenic gasolines which generate sulfurdioxide in automobile exhausts. The nitrogen-containing compoundspresent in these products are familiar from their harmful effects on theactivity and life of the catalysts used.

Furthermore, these refractory compounds are well known in industry owingto the difficulty of removing them. However, it is becoming increasinglynecessary to remove these compounds from light hydrocarbons because,notably for the sulfur-containing refractory compounds, theenvironmental restrictions, in Europe as well as in the USA or Japan,and particularly restrictions on sulfur emissions to atmosphere, arebecoming more and more stringent. At present, the maximum permitted ingasolines is 150 ppm of total sulfur. However, international agenciesare asking for sulfur in gasolines to be limited to less than 50 ppm oftotal sulfur by 2005 and envisage restriction to less than 10 ppm oftotal sulfur from 2009 for all products. In thiophenic hydrocarbons, forexample gasolines or naphthas, these contents are well above 100 ppm andare generally between 100 and 1000 ppm of total sulfur.

Mixing with desulfurized and denitrogenated gasolines is the only way ofdisposing of the stocks of gasolines obtained from fluid catalyticcracking (FCC) or of pyrolysis gasolines.

To remove thiophene and its derivatives from thiophenic gasolines, itwas proposed, in U.S. Pat. No. 6,338,788, to extract the thiophenecompounds from the charge by mixing the latter with an electrolyte and asolvent. The resulting mixture is fed into an electrochemical cell, soas to oligomerize the thiophene compounds. The oligomers are removedfrom the charge subsequently. The electrolyte/solvent mixture can berecovered and recycled in a new mixture with the charge to be treated.The preferred solvents are generally compounds that are able to formcomplexes with the aromatic compounds present in the hydrocarbons, suchas alkylene carbonates, benzonitriles, sulfolanes or derivatives ofmorpholine. Salts that are used are the tetraalkylammonium salts, suchas fluoroborates, fluorophosphates or halides. Apart from the difficultyof implementing this technique with an electrochemical cell on anindustrial scale, such a method becomes prohibitive for refiners, whenit is necessary to purchase the required solvents and electrolytes, andin addition recycle them for reasons of environmental protection.

Another possible solution is the one disclosed in U.S. Pat. No.4,188,285. There it is proposed to remove the thiophenes from gasolinesby bringing gasoline from C₅ to C₇ into contact with a catalystcomprising a zeolite of the faujasite Y type exchanged with silver, at atemperature between 20 and 370° C., and an hourly space velocity between0.1 and 20. Here, the silver atom is exchanged on the faujasite. In suchan operation, the proportion of olefins remains unchanged before andafter treatment of the gasoline. In that patent, it is a question ofadsorbing the thiophene and its alkylated derivatives on zeolite Yexchanged with Ag⁴ and Cu²⁺ ions utilizing complexing effects usingformation of n bonds, the copper being reduced to Cu⁺ as described byRalph T. Yang et al., in Science & Technology, Vol. 301, p. 79, and inInd. Eng. Chem. Res. 2001, 40,6236-6239, or by A. Hernandez-Maldonado etal., in Ind. Chem. Res. 2003, 42, 3103-3110.

The present applicant has investigated a method of purification of lighthydrocarbons containing sulfur-containing and/or nitrogen-containingcompounds that are refractory to catalytic hydrofining treatments, whichaims to make these compounds heavier by oxidative polymerization of thelatter, so that they can be removed more easily from these hydrocarbons.In this method of purification, the applicant aims to achieve not onlydesulfurization and denitrogenation, but also regeneration of theactivity of the oxidizing agent used, by combining the reaction ofoxidation with a process of regeneration and activation of the oxidizingagent used.

The present invention therefore relates to a method of purification oflight hydrocarbons with cut point between 20 and 250° C., containingsulfur compounds and/or nitrogen compounds that are refractory to theusual hydrofining treatments, characterized in that it comprises

-   (a) a stage of oxidative polymerization of the compounds comprising    an —X—CH═ group in a hydrocarbon ring with 5 to 6 ring members,    where X represents a sulfur atom or a nitrogen atom, in the presence    of at least one oxidizing agent selected from the metal cations,-   (b) a stage of separation of the polymers formed and of the    oxidizing agent with the light hydrocarbons, and-   (c) a stage of oxidation of the metal cation,

these stages being carried out in that order, it being possible for eachof these stages to be combined with at least the next stage.

Within the scope of the present invention, the compounds containing an—X—CH═ group in a 5-6-membered hydrocarbon ring are thiophene compounds,ranging from thiophene to its alkylated or aralkylated derivatives, andpyrrole compounds, ranging from pyrrole to its alkylated or aralkylatedderivatives, generally present in the hydrocarbons and constitutingproducts that are refractory to desulfurization and/or denitrogenationby conventional treatments of catalytic hydrogenation.

To carry out the invention, the metal cations are a introduced in liquidform, dispersed or dissolved in an aqueous or organic liquid, orsupported on a solid. The method according to the invention is thereforea multiphase process with two or three phases, depending on whether ornot the metal cations are deposited on a solid support before the startof the reaction employed in this method, namely a polymerization of thesulfur compounds and/or nitrogen compounds.

Within the scope of the present invention, when the metal cations areimmobilized on a solid support in the fixed or moving bed, thepolymerization reaction takes place starting at room temperature, underatmospheric pressure, at an hourly space velocity (HSV) of at least 0.1h⁻¹.

In general, for the metal cations to polymerize the sulfur compoundsand/or nitrogen compounds, the oxidizing metal cation must have a redoxpotential greater than that of the molecule to be oxidized/polymerizedin the reaction mixture.

To achieve redox potentials such as permit the polymerization of thethiophene or pyrrole compounds, the metal cation is selected from thecations of metallic elements of the group comprising iron, copper,molybdenum, cerium, manganese and vanadium, and each of these metalsmust be present in the reaction mixture with a degree of oxidation of atleast 2. These metal cations are used in the form of salts of the groupcomprising the halides, nitrates, citrates, carboxylates, phosphates,sulfates, persulfates, borates, perborates and the bidentate andpolydentate complexes of linear or cyclic form, containing atoms ofnitrogen, sulfur and/or oxygen as the coordinating element. By bidentateand polydentate complexes we mean, non-limitatively, thephthalocyanines, porphyrins, cyclames, bipyridines and Saler complexes.

When the metal cation is introduced in the dispersed state or insolution in water, the polymerization reaction is a liquid/liquid(organic/aqueous) two-phase reaction, and the polymers formed and theoxidizing cations can be removed by the decanting of separated phases,by filtration and/or extraction by techniques that are well known to aperson skilled in the art.

In another embodiment of the invention, the method is carried out in thepresence of a solid selected from the group comprising charcoal, clays,zeolites, molecular sieves, amorphous aluminosilicates, alkalinesilicates, silicoborates, silica-magnesias, and aluminophosphates. Thissolid can support the salts of the metal cations required for theinvention, whether or not there is ionic interaction between thesecations and these solids.

In a preferred embodiment, the protons initially present on the supportwere exchanged for metal cations, then these metal cations were oxidizedbefore use, making it possible to obtain an oxidation state of thesemetals greater than or equal to two. This oxidation state is essentialfor the polymerization reaction to take place in the hydrocarbons, ashad already been found by Bein for media that are less complex than thehydrocarbons obtained from petroleum distillation, in his article inStudies in Surface Science and Catalysis, Vol. 102, 1996, pp. 295-319.

The advantage of a method employing the metal cation in the form ofcounter-ion of a solid support is that it is possible for the reactionof polymerization to be carried out in the usual conditions of refining,i.e. with a catalyst bed of the types used in refining. Anotheradvantage is that it is possible to envisage in-situ or ex-situregeneration of the metal cations used as oxidizing agent.

The following may be chosen as support of the metal cations: crystallineor amorphous solids, cation exchangers, containing at least one metalfrom the group of elements comprising silicon, aluminum, zirconium,titanium, germanium, gallium and boron, used alone or in combination,and with specific surface of a at least 10 cm²/g.

Preferably, these supports are selected from the clays, including thebentonites, and the zeolites, including Sapo, Alpo and Beta, andmesoporous, for example of the type MCM 41, molecular sieves, amorphousaluminosilicates, alkaline silicates, silicoborates, silica-magnesias,these solids having a pore size between 1.5 nm and 200 nm.

To obtain these supported cations, it is necessary to bring the solidinto contact with metal cation salts in the form of an aqueous ororganic solution, the salts being selected from the nitrates,carboxylates, sulfates, persulfates, citrates, phosphates, borates,perborates and halides of metals, including iron, copper, molybdenum,manganese, vanadium and cerium. The preferred salts are selected fromferric chloride, cuprous chloride, molybdenum chloride, vanadiumoxychloride and cerium chloride.

Preferably, the amount of metal cation present on the support can varyfrom 0.1 wt. % to 30 wt. % of the metal corresponding to said cation.

These supported cations can exert their action in a fixed-bed,moving-bed or fluidized-bed process or in suspension in a liquid.

In the course of polymerization of the sulfur compounds and/or nitrogencompounds, the polymers formed are entrained in suspension in thehydrocarbon or deposited on the solid. They can therefore be extracted,decanted, filtered or even distilled, in order to be removed from thehydrocarbon thus purified. When the polymers formed are deposited on thesolid, the removal stage comprises extracting the polymers deposited onthe support by washing with solvent, notably by charging, by desorptionby a stream of inert gas selected from helium, nitrogen, carbon dioxideand water vapour, at a temperature above 100° C., and/or by combustionby injecting air or oxygen, preferably after removal of the lighthydrocarbons still present on the particles of support.

To restore or maintain the supported cation in an oxidation statesufficient for the reaction of polymerization to take place normally,the metal cation is oxidized. This stage of oxidation of the metalcation, whether or not it is supported, comprises restoring the metalcations to a degree of oxidation of at least 2, by oxidation, byinjecting air or liquids containing peroxides or other metal cationsthat are more oxidizing, optionally simultaneously increasing thetemperature of the oxidizing agent.

In a preferred embodiment of the invention, it is possible for thestages used alone or in combination to be combined in a continuousprocess or a batch process. Thus, at the end of the stage of oxidationof the metals, the oxidized metal cation is reused directly for a newstage of oxidative polymerization. Moreover, carrying out certain stagesof the method, for example the first stage of oxidative polymerizationand the second stage of removal of the polymers obtained, which arepresent in the liquid phase and/or on the support, when a support isused, would remain within the scope of the invention. The same wouldapply if we combine the stage of removal of the polymers obtained andthe third stage of oxidation of the metal cation. An embodiment of themethod simultaneously combining the third and the first stage, or eventhe three stages depending on the type of fixed or moving bed that canbe employed, also falls within the scope of the present invention.

Another object of the invention is the application of this method to thefinishing treatment of industrial streams containing refractorysulfur-containing or nitrogen-containing compounds. More particularly,this method can be used for the desulfurization/denitrogenation ofgasolines produced by catalytic cracking and of effluents from the steamcracking plant, notably pyrolysis gasolines. This method can also beapplied as finishing treatment for aromatic effluents such as benzene,toluene and xylene.

The following example is given for the purpose of illustrating theinvention, though without wishing to limit its scope.

EXAMPLE

The present example describes several embodiments of the method of theinvention, using various oxidizing cations, and their efficiency withrespect to desulfurization and/or denitrogenation.

Test I:

FeCl₃ powder is suspended in a gasoline from catalytic cracking or LCCSby mixing at a temperature of 25° C. The Fe/S ratio (total sulfur in theLCCS) is 16 atoms of Fe per atom of sulfur (16 atoms/atom).

Test II:

Anhydrous FeCl₃ is deposited on silica: the supported cation thus formedis mixed with LCCS at 40° C. The Fe/S ratio is 13 atoms/atom.

Test III:

Anhydrous FeCl₃ is deposited on activated charcoal: the supported cationthus formed is mixed with LCCS at 40° C. The Fe/S ratio is 13atoms/atom.

Test IV;

A zeolite β is charged with sodium, in the form of particles from 0.15to 0.5 mm, exchanged with copper acetate, in a tubular reactor, and LCCSis circulated through it at an hourly space velocity of 1.2 h⁻¹ and at atemperature of about 25° C., at atmospheric pressure. The effluent isanalyzed for sulfur and/or nitrogen after circulation for 3 hours andafter 15 hours.

Test V:

A zeolite β is charged with sodium in the form of particles from 0.15 to0.5 mm, exchanged with copper acetate, in a tubular reactor, and LCCS iscirculated through it at an hourly space velocity of 1.2 h⁻¹ and at atemperature of about 150° C., at atmospheric pressure. The effluent isanalyzed for sulfur and/or nitrogen after circulation for 1 h 30 min andafter 14 hours.

Test VI:

Test V is repeated four times, with each test lasting 7 hours. Thesupported cation is reactivated in accordance with stages 2 and 3described above, these stages being simultaneous and carried out withcirculation of air, for 5 hours, at 350° C.

The zeolite β is charged with sodium exchanged with copper II thusreactivated in a tubular reactor and LCCS is again circulated through itat an hourly space velocity of 1.2 h⁻¹ and at a temperature of about150° C., at atmospheric pressure. The effluent is analyzed for sulfurand/or nitrogen after circulation for 1 h 30 min and after 3.5 hours.

Test VII:

A zeolite β initially in protonated form, with particle size varyingfrom 0.15 to 0.5 mm, is exchanged with copper acetate, and is then mixedwith LCCS at a temperature of about 40° C., at atmospheric pressure. TheCu/S ratio is 0.96 atom/atom. The effluent is analyzed for sulfur and/ornitrogen after 7 hours.

Test VIII:

A zeolite β with sodium in the form of particles from 0.15 to 0.5 mm,exchanged with copper acetate, is mixed with LCCS at 40° C. The Cu/Sratio is 10.3 atoms/atom. The effluent is analyzed for sulfur and/ornitrogen after 6 hours.

Test IX:

A zeolite β with sodium in the form of particles from 0.15 to 0.5 mm,exchanged with copper acetate, is mixed with LCCS at 40° C. The Cu/Sratio is 30.8 atoms/atom. The effluent is analyzed for sulfur and/ornitrogen after 5 hours 30 minutes.

Test X:

A zeolite β with sodium, in the form of particles from 0.15 to 0.5 mm,exchanged with copper acetate, is mixed at 40° C. with a model fluidcontaining 0.5 wt.% thiophene, 0.5 wt. % dodecane and 99 wt. % toluene.The Cu/S ratio is 1.5 atom/atom. The effluent is analyzed for thiopheneand mercaptans after 6 hours 30 minutes.

Test XI:

An emulsion is prepared at room temperature from 202 g of organicsolution containing 99 wt. % toluene, 0.5 wt. % pyrrole and 0.5 wt. %n-decane, with 112 g of aqueous solution of FeCl₃ at 6.4 wt. %. The Fe/Nratio is 2.94 atoms/atom. Analyses for total nitrogen are carried outafter 5 hours.

Test XII:

An emulsion is prepared at room temperature from 200 g of organicsolution containing 99 wt. % toluene, 0.5 wt. % pyrrole and 0.5 wt. %n-decane, with an aqueous solution of Ce(SO₄)₂ at 30 wt. %. The Ce/Nratio is 4.7 atoms/atom. Analyses for total nitrogen are carried outafter 40 minutes.

Test XIII:

4.3 g of anhydrous powder of FeCl₃ is dispersed in 183 g of a solutioncontaining 99.25 wt. % toluene, 0.5 wt. % thiophene and 0.25 wt. %dodecane, at 30° C. The Fe/S ratio is 2.45 atoms/atom. Analyses fortotal sulfur are carried out after 2 hours.

Test XIV:

A zeolite β in powder form, initially in protonated form, is exchangedwith copper acetate, and is then mixed with 200 g of a solutioncontaining 99.25 wt. % toluene, 0.5 wt. % thiophene and 0.25 wt. %dodecane, at a temperature of about 40° C. and at atmospheric pressure.The Cu/S ratio is 0.8 atom/atom. Analyses for total sulfur are carriedout after 4 hours.

The results obtained in desulfurization and denitrogenation are given inthe following table, in which the contents of sulfur and of nitrogen areexpressed in ppm. TABLE S(total) S(thiophene) S(2 + 3methylthiophene)N(total) Test (inlet) (outlet) (inlet) (outlet) (inlet) (outlet) (inlet)(outlet) I 121 94 60 54 50 35 — — II 121 80 60 49 50 35 — — III 121 9760 46 50 46 — — IV 113 70 53 28 50 36 — — 113 93 53 44 50 36 — — V 12131 57 23 53 6 — — 121 76 57 35 53 37 — — VI 105 46 — — — — — — 105 70 —— — — 17 1.7 VII 215 166 39 26 122 117 — — VIII 121 84 60 40 50 36 — —IX 121 93 57 36 53 49 — — X — — 2074 1064 — — — — XI — — — — — — 1050597 XII — — — — — — 1050 177 XIII — — 1900 733 — — — — XIV — — 1900 468

1. A method of purification of light hydrocarbons with cut point between20 and 250° C., containing sulfur compounds and/or nitrogen compoundsthat are refractory to the usual hydrofining treatments, characterizedin that it comprises (a) a stage of oxidative polymerization of thecompounds comprising an —X—CH═ group in a hydrocarbon ring with 5 to 6ring members, where X represents a sulfur atom or a nitrogen atom, inthe presence of at least one oxidizing agent selected from the metalcations, (b) a stage of separation of the polymers formed and of theoxidizing agent with the light hydrocarbons, and (c) a stage ofoxidation of the metal cation, these stages being carried out in thatorder, it being possible for each of these stages to be combined with atleast the next stage.
 2. The method as claimed in claim 1, characterizedin that the metal cations are introduced in liquid form, dispersed ordissolved in an aqueous or organic liquid, or supported on a solid. 3.The method as claimed in one of the claims 1 and 2, characterized inthat the oxidizing metal cation has a redox potential greater than thatof the molecule to be polymerized in the reaction mixture.
 4. The methodas claimed in one of the claims 1 and 2, characterized in that the metalcation is a cation of a metallic element of the group comprising iron,copper, molybdenum, manganese, cerium and anadium, with a degree ofoxidation of at least
 2. 5. The method as claimed in one of the claims 1to 4, characterized in that the metal cation is used in the form ofhalide, nitrate, citrate, carboxylate, phosphate, sulfate, persulfate,borate, perborate, bidentate and polydentate complex of linear or cyclicform, comprising atoms of nitrogen, sulfur and/or oxygen as thecoordinating element.
 6. The method as claimed in one of the claims 1 to5, characterized in that when the metal cation is introduced dispersedor in solution in water, the reaction of polymerization is two-phase,and the polymers formed and the oxidizing cations are removed bydecanting, filtration and/or extraction.
 7. The method as claimed inclaim 6, characterized in that the reaction of polymerization is carriedout in the presence of a solid selected from the group comprisingcharcoal, clays, zeolites, molecular sieves, amorphous aluminosilicates,alkaline silicates, silicoborates, silica-magnesias, andaluminophosphates.
 8. The method as claimed in one of the claims 1 to 7,characterized in that the metal cation is supported on acation-exchanger crystalline or amorphous solid, containing at least onemetal of the group of elements comprising silicon, aluminum, zirconium,titanium, germanium, gallium and boron, alone or in combination, andwith specific surface of at least 10 cm²/g.
 9. The method as claimed inclaim 8, characterized in that the solid is selected from the clays,including the bentonites, the zeolites, including the Sapo, Alpo andBeta and the mesoporous zeolites, molecular sieves, amorphousaluminosilicates, alkaline silicates, silicoborates andsilica-magnesias, this solid having a pore size varying from 1.5 nm to200 nm.
 10. The method as claimed in claims 8 and 9, characterized inthat the supported metal cation is obtained by bringing the solid intocontact with metal cation salts in the form of an aqueous or organicsolution, the salts being selected from the nitrates, carboxylates,sulfates, persulfates, borates, perborates, citrates, phosphates andhalides of metals, including iron, copper, molybdenum, manganese,vanadium and cerium.
 11. The method as claimed in claim 10,characterized in that the metal salt is selected from ferric chloride,cuprous chloride, molybdenum chloride, vanadium oxychloride and ceriumchloride.
 12. The method as claimed in one of the claims 8 to 11,characterized in that the supported metal cation contains from 0.1 wt. %to 30 wt. % of the metal corresponding to said cation.
 13. The method asclaimed in one of the claims 8 to 12, characterized in that thesupported cation is used in a fixed bed, in a moving bed, in a fluidizedbed or in suspension in a liquid.
 14. The method as claimed in one ofthe claims 8 to 13, characterized in that the stage of removal of thepolymers deposited on the solid supporting the cation comprisesextracting these polymers by washing with the solvent, notably bycharging, by desorption by an inert gas stream selected from helium,nitrogen, carbon dioxide and water vapour, at a temperature above 100°C., and/or by combustion by injecting air or oxygen, preferably afterremoval of light hydrocarbons that are still present in the solidsupport.
 15. The method as claimed in claims 8 to 14, characterized inthat the stage of polymerization of the refractory compounds is followedby removal of the polymers formed that are present in the treatedhydrocarbon, either by decanting, or by filtration, or by solventextraction, or by distillation.
 16. The method as claimed in one of theclaims 1 to 15, characterized in that the stage of oxidation of themetal cation, whether or not it is supported, comprises restoring themetal cations to a degree of oxidation of at least 2 by oxidation, byinjecting air or liquids containing peroxides or other more-oxidizingmetal cations, and optionally simultaneously increasing the temperatureof the oxidizing agent.
 17. The method as claimed in one of the claims 1to 16, characterized in that the stages of the method, takenindividually or in combination, are combined in a continuous process ora batch process.
 18. The method as claimed in one of the claims 1 to 17,characterized in that the metal cation, whether or not it is supported,is reused in the first stage of the method.
 19. An application of themethod as claimed in one of the claims 1 to 18 as a finishing treatmentfor industrial streams containing refractory sulfur and/or nitrogencompounds.
 20. The application as claimed in claim 19 to FCC gasolinesand to effluents from the steam cracking plant, notably to pyrolysisgasolines.
 21. The application as claimed in claim 19, as a finishingtreatment for aromatic effluents such as benzene, toluene and xylene.